Liquid helium 4 exists as a classical liquid (He I) above the lambda transition and as a quantum liquid (He II) below the lambda transition. The lambda transition temperature is 2.172 Kelvin (K.) at 0.00497 MPa. He II has several unique and useful properties such as an enormous effective heat conductivity. Refrigeration systems operating to temperatures below the helium 4 lambda transition temperature are of interest because of the enhanced heat transfer in He II as well as improved lower temperature performance in superconducting materials.
At present, refrigeration systems, including those for cryogenic applications, are almost entirely based on successive compression and expansion cycles of a gas. Generally, the efficiency of practical gas cycle refrigerators is only a fraction of the ideal Carnot cycle efficiency, and the efficiency generally decreases with a decrease in the size of the refrigerator. The efficiency of gas cycle refrigerators is particularly low at cryogenic temperatures, e.g., in the 2 K. to 20 K. range. Reliability can also be a problem with large refrigeration systems operating to about 2 K.
A typical gas cycle refrigeration system that operates down to He II temperatures has a 1.8 K. static, sub-cooled He II bath that cools a superconducting coil. The bath results from heat exchange with He II at saturated vapor pressure. Several cold compressors compress the low pressure, cold helium vapor from the saturated He II before the cold gas passes through low pressure heat exchangers to room temperature where additional compressors raise the pressure to 0.1 MPa. The vapor is then recirculated in the refrigeration system. Because of the extensive subatmospheric sections in the flow loop, leaks are a source of reduced reliability. The use of a gas cycle system to cool to He II temperatures also requires large volumes and weights for pumps, compressors, and other equipment.
It has long been known that certain magnetic materials exhibit the magnetocaloric effect: they increase in temperature when placed in a magnetic field and decrease in temperature when removed from the field. Application of a magnetic field to such solid magnetic materials is analogous to compressing a gas (producing an increase in temperature) and removing the field from the solid is analogous to expanding a gas (Producing a decrease in temperature). Thus, it has been recognized that a thermodynamic refrigeration cycle can be achieved using a magnetic material as the working material in a manner analogous to the refrigeration cycles of a gas. Refrigerators utilizing the magnetocaloric effect require several essential components. A magnetic material that exhibits a magnetocaloric effect suited to the intended operating temperature range is the refrigerator's working material. Magnets of sufficient field strength to produce the necessary field changes at the working material are required. Means for effecting the necessary cyclic changes in magnetic field at the working material must be included. Switches enabling heat transfer and heat transfer modes to transfer heat to and from the working material at requisite locations within the refrigerator are necessary. A thermal source from which heat is extracted is necessary, as is a sink to which heat is rejected. Finally, a structure with appropriate thermal, magnetic, and physical properties to support the essential elements of the refrigerator with minimum negative performance impact must be included. Examples of relatively recent designs proposed for magnetic refrigerators are shown in U.S. Pat. Nos. 4,033,734, 4,069,028, 4,107,935, 4,332,135, 4,392,356, 4,408,463, 4,441,325, 4,457,135, 4,459,811, 4,464,903, 4,507,927, 4,507,928, and 4,702,090.
U.S. Pat. No. 4,702,090 to Barclay et al. suggests a design illustrating a realization of these necessary elements. In this refrigerator the magnetic working material is gadolinium gallium garnet (Gd.sub.3 Ga.sub.5 O.sub.12), a material suitable for magnetic refrigerators operating over any temperature range between about 1 K. to about 20 K. The working material is attached to a peripherally driven, bearing-mounted wheel. The wheel rotates between the fins of an element called the thermal extractor. Each thermal extractor fin consists of eight equal-sized circular segments of alternating copper and stainless steel. The copper segments are attached to two centrally located copper thermal bus bars of square section. Copper segments attached to the same bus bar are separated by copper segments attached to the other bus bar. The bus bars stand perpendicular to the plane of the wheel's rotation and contain the symmetry axis of the refrigerator. The wheel, its supporting bearing and the thermal extractor fins are contained in a hermetically sealed housing which contains gaseous helium. The peripheral drive mechanism penetrates the housing, as do the two bus bars. Seals are located at these penetrations.
The required magnetic field is produced by two symmetrically placed pairs of magnets, each pair with the housing between them. The magnets are attached to a spherical structure enclosing the magnets and the housing and supporting the large attractive forces between the magnets. The magnet pairs are located such that copper thermal extractor fin segments attached to the same thermal bus bar are between them, within the housing. The field change necessary for refrigeration is produced by rotating the wheel containing the magnetic material through the stationary field produced by the magnet pairs. The material's temperature is highest in the high field regions and lowest in the low field regions. The helium within the housing acts as a heat transfer medium conducting heat between the working material and the thermal extractor. The two copper segments at the high field regions and the thermal bus bar to which they are attached are at the refrigerator's highest temperature. Heat rejection from the refrigerator occurs by conduction through these elements. Similarly, the two copper segments at the low field regions and the thermal bus they are attached to are at the refrigerator's lowest temperature. The refrigerator absorbs heat by conduction through these elements. No heat transfer occurs while the working material traverses the regions between the stainless steel segments because the thermal conductivity of steel is very small. Thus, switching enabling heat transfer is achieved by providing a suitable environment at locations where heat transfer is to occur.
The performance of this refrigerator may be limited because of the need to use gaseous helium in the housing to transfer heat across the gap between the moving solid magnetic working material and the solid copper of the thermal extractor. The thermal conductivity of helium gas is the smallest of any material in the refrigerator. To achieve maximum heat transfer very small gaps between the magnetic material and the thermal extractor must be maintained. This leads to potential difficulties in fabrication, assembly and operation due to tolerance accumulation, thermal contraction effects and component wear.
A magnetic refrigerator with thermal conduction as the heat transfer mode between its components, and operating in the range between 1.8 K. and 4.7 K., as would be required in the production of He II, must maintain a minimum clearance gap between the magnetic material exhibiting the magnetocaloric effect and solid copper heat exchangers transferring heat into and out of the refrigerator. In prior art magnetic refrigerators, the minimum clearance has proven difficult to maintain in the fabrication and assembly of the refrigerators.
The efficacy of the heat transfer modes into and out of the magnetic refrigerator are critical to refrigerator performance. Three primary heat transfer mechanisms are conduction in a gas or solid, convection of a gas or liquid, and phase change of a fluid, e.g. boiling or condensation. The simplest refrigerators use conduction, which also yields the most severe performance limitation of the three mechanisms. The unit described above, suggested by U.S. Pat No. 4,702,090, utilizes conduction through solid copper buses and helium gas for heat transfer into and out of the refrigerator. Heat transfer here is limited by the thermal conductivity of copper and helium gas in the 1.8 K. to 15 K. temperature range and the intercomponent gap. The convective heat transfer mechanisms of some proposed magnetic refrigerators require the pumping of a gas or liquid for heat transfer. Phase change mechanisms are limited to specific temperatures where phase changes occur, e.g. 4.2 K., for normal boiling liquid helium. As an alternative to a single heat transfer mechanism in magnetic refrigerators, it has been proposed to use high conductance heat pipes which transport heat by an evaporation-condensation cycle of fluid; helium or hydrogen have been proposed for refrigerators operating below 20 K.
The use of high field superconducting magnets to produce the most efficient magnetic refrigeration imposes substantial mechanical loads on the support structure of the refrigerator. The support structure must be sufficiently massive and rigid to resist the forces imposed without substantial deformation, and yet must not constrain the mechanical operation of the magnetic refrigerator or create a significant thermal addenda with consequent loss of refrigeration efficiency, and must not permit undue heat transfer between hot and cold regions.